How long are your lights on at the same intensity?
If all day this may be the problem.
Even when trying grow plants, light can vary.
In my current tank , sun comes in around 8AM, remains about 3 hours, then the tank in in the shade until about 3PM when it gets another 2 hours of moving sun.
The plants grow like crazy, and algae is almost non existent (using sand as substrate
View attachment 1390274
morning sun above, afternoon below.
View attachment 1390275
from about 11 AM to 3 PM the tank is lit with only ambient light, like the pic below.
View attachment 1390276
By the way Goyder River and Murry River Rainbows are often available these days, there may be others.